Such models make a clear prediction that the inner radius of the cool
disc should recede as the LHS drops in luminosity. This can in
principle be tested using the colour temperature and luminosity of the
cool disc component to estimate its radius
(e.g. [1]}; [2]}). However, such observations
are complicated by the fact that the disc is at low temperature at low
luminosities, \(\sim 0.3\)  keV for a \(10M_\odot \)  black hole at \(0.02L_{Edd}\) , even if the disc extends down to \(6R_g\)  (where
\(R_g=GM/c^2\) ). Such low energy X-rays cannot be seen with the 3 keV
bandpass limit of RXTE, the satellite which has accumulated the
most BHB data to date ([3]}; [4]}). CCD
observations can extend the bandpass down to lower energies, but these
are still not straightforward to interpret as the disc is not the
dominant spectral component in this state. Instead, the hard X-ray
emission dominates the spectrum, so irradiation can change the disc
temperature ([5]}, hereafter GDP08), while
the disc luminosity can be underestimated due to photons which are
Compton upscattered into the hard X-ray spectrum [6]}.
This component is still not easy to study even with CCD
detectors since low energy X-rays are absorbed by interstellar
gas. Most BHB are in the galactic plane so have gas columns of \(N_H\ge 10^{22}\)  cm\(^{-2}\)  which effectively block emission below 1 keV.
